Method for biological effluent treatment

ABSTRACT

The invention relates to a method of biological treatment of wastewater by means of activated sludge, wherein the wastewater is introduced first into an aerated activated sludge tank (B tank) and then by turns into one of several sedimentation and recirculation tanks (SU tanks), which are permanently linked with said B tank and in which a separation of activated sludge and clear water occurs, and after separation activated sludge is returned to the B tank and clear waters is drawn off; several times a day, in the SU tanks an operating cycle proceeds comprising a stirring phase (R phase), a pre-settling phase (V phase) and a discharge phase (A phase). In the R phase the activated sludge is remixed with the water, in the V phase the activated sludge settles down and in the A phase clear water is drawn off (single basin technique). The cycles in the SU tanks are phase displaced to each other and the A phases are adjacent, so only in the A phases the SU tanks are flown through, an approximately constant water level is present; thus causing an outflow from the purification plant which corresponds to its inflow (throughflow principle). Before the R phase settled and thickened activated sludge is reintroduced from the SU tank into the B tank.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for biological purification ofmunicipal or like wastewater by means of activated sludge, wherein thewastewater is introduced first into an aerated activated sludge tank (Btank) and then by turns into one of several sedimentation andrecirculation tanks (SU tanks), which are permanently linked with said Btank and in which, several times a day an operating cycle proceedscomprising a stirring phase (R phase), a pre-settling phase (V phase)and a discharge phase (A phase), wherein in turns in the R phase theactivated sludge is remixed with the water, in the V phase the activatedsludge settles down and in the A phase clear water is drawn off andwherein the cycles in the SU tanks are phase-displaced to each other andthe A phases are adjacent, only in the A phases the SU tanks are flownthrough, an approximately constant water level is present, thus causingan outflow from the purification plant corresponding to its inflow(throughflow principle).

2. Description of the Prior Art

From the European patent application EP 968 965 a method for biologicalwastewater purification by means of activated sludge is known where thewastewater is introduced first into an aerated activation tank and theninto a settling tank, in which a separation of activated sludge andclear water occurs and after separation activated sludge is returnedinto the activation tank and clear water is discharged. Several times aday an operating cycle is performed, comprising a stirring phase, asecondary settling phase and a discharge phase, wherein in the stirringphase the activated sludge is remixed with the water, in the secondarysettling phase the activated sludge settles down and in the dischargephase clear water is drawn off. According to the above method of priorart, the purification is done in a biological two-tank system—theactivation and the sedimentation tanks with continuous inflow andintermittent outflow. In the intervals without outflow the water levelrises due to the inflow (damming-up principle). The patent claim of thismethod consists in that after the pre-settling phase and before thestirring phase settled activated sludge is returned into the activationtank of the “two-tank system with damming-up operation”. That thismethod relates to a damming-up operation can be seen from thedescription of the document (pages 14 and 15) as well, stating: “thatwater is introduced permanently to the first region and from there itspills into the second region. A discharge of purified drain water isperformed here only during the third step of the method. During theother steps the drain water accumulates in both regions or—in the caseof presence of an anaerobic pre-treatment—also in this region.” Also inclaim 1 it is clearly evident that it concerns “two-tank systems withdamming-up operation” which “are connected in parallel and operatedtime-delayed.” This method of prior art is very suitable for smallpurification plants. For middle or large-scale purification plants,however, it is far better to use the throughflow principle. Then theoutflow from the purification plant corresponds to the inflow.

A similar method is known from the WO 97/08104, where at the beginningof each cycle the same sludge concentration is adjusted in theactivation and sedimentation tanks, the reintroduction of thenon-settled activated sludge occurring during the stirring phase. Areintroduction of settled and well-thickened activated sludge before thestirring phase is not provided for.

Furthermore, a similar method is known from the European patent EP 0 670817 B1 of Dec. 29, 1999, where the wastewater is treated in two cells,wherein the wastewater is aerated an mixed in the treatment anddischarge cell and wherein the reintroduction of sludge from thetreatment and discharge cell into the first treatment cell occurs duringthe mixing period (B and R phase). Here it is essential that cellaeration and mixing is done in the treatment and discharge and nosettled and thickened activated sludge is let to reintroduction, whichis why a longer time is needed for the reintroduction and a smallercontent of dry substance in the first treatment cell is achieved, thus aloss in time for the other phases comes about (compare claim 1 of thedocument).

A similar method is known from the European application EP 1 110 916 ofJan. 17, 2000. In a purification plant operating according to thethroughflow principle and exploiting the one-basin technology, settledand thickened activated sludge is returned after the V phases and beforethe R phases into the first treatment tank. The reintroduction of sludgeis done in a relatively short time, which makes a large return amountnecessary.

The EP 0 399 013 relates to a facility for wastewater processing, inwhich buffering of larger amounts of wastewater (wastewater impacts) ispossible in a simple way; this is achieved in that the closure means ofthe outlet of the activation tank comprising a moveable closure bodymade from an elastically deformable foil. The fluid from the activationtank is transported into the secondary clearing tank by means of aair-jet lift. On the bottom of both tanks a closeable opening isprovided which serves for transporting settled sludge from the secondaryclearing tank into the activation tank and which is opened only for ashort time. Thus, the connection of the two tanks is hydraulicallyinterrupted and causes different water level positions in the two tanks.The method underlying this facility is an activation method operatingaccording to the damming-up principle with intermittent, short-timereintroduction of sludge from the post-clearing tank into the activationtank, as opposed to the throughflow method as cited in the beginning.

SUMMARY OF THE INVENTION

The invention is based on the problem to improve the above-describedmethods for biological wastewater purification in a manner that allowsapplication in middle and large-scale purification plants by using thethroughflow principle and, at the same time, achieving a higher sludgeconcentration in the activation tank at a shorter reintroduction timethrough the reintroduction of settled and well-thickened activatedsludge. This problem is solved by the features of claim 1, according towhich after the V phases and before the R phases, settled and thickenedactivated sludge is reintroduced into the B tank from the SU tanks (Sphase).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: a schematic representation of the individual phases during acycle,

FIG. 2: a schematic representation of a duplex siphon for the transportof fluids in both directions,

FIG. 3: a schematic representation of the individual phases during acycle using a duplex siphon, and

FIG. 4: a schematic representation of a clear water outlet (air pressureclosure).

DETAILED DESCRIPTION OF THE INVENTION

The invention is distinguished in that, in order to achieve thethroughflow principle, the activation tank (B tank) is permanentlyconnected hydraulically with several sedimentation and recirculationtanks (SU tanks), wherein in the SU tanks, several times a day, anoperating cycle proceeds comprising a stirring phase (R phase), apre-settling phase (V phase) and a discharge phase (A phase). In the Rphase the activated sludge is remixed with the water, in the V phase theactivated sludge settles down and in the A phase clear water is drawnoff. The cycles in the SU tanks are phase displaced in such a mannerthat the A phases are adjacent to each other, thus causing an outflowfrom the purification plant which corresponds to its inflow (throughflowprinciple). In this context it is essential that before the R phase,settled and well-thickened activated sludge is reintroduced into the Btank (S phase). Advantageously, a high sludge concentration in the Btank and a short return time is achieved in the case when thereintroduction is done only after termination of the draw-off phase ofclear water (A phase).

The activated sludge being reintroduced is suitably taken from thebottom of the SU tank since there will occur the highest sludgeconcentration.

By the reintroduction of the settled sludge, water is displaced in the Btank, which water is returned to the SU tank via an opening near to thesurface. This water also contains activated sludge, however, in a lesserconcentration as compared to the returned settled sludge. In order tominimize this sludge backflow, it is suitable according to the presentinvention to interrupt or throttle the aeration in the activation tankbefore starting the reintroduction of the activated sludge. By thismeasure the activated sludge whirled up by the aeration sinks untilbelow the level of the surface-near opening, and the sludgeconcentration of the displaced water is reduced.

The surface-near openings are provided with flaps opening automaticallyand being closed in the V and A phases.

The reintroduction of settled sludge can be done with electrical devices(pumps, stirring devices) or by means of air-jet lifts.

The stirring in the SU tanks (R phase) can be done by several ways aswell. Air may be by injected, electrically driven stirring apparatus maybe used or air-jet lift.

For the reintroduction of the sludge and stirring in the SU tanks acombined air-jet lift according to FIG. 2 (duplex siphon) may beemployed. In the case of presence of a fine-bubble aeration for the Btank this aeration can be turned off and the pressured air thusavailable can be employed for the operation of the duplex siphon. Inthis case it is important that for the stirring such a strong jet ofwater is generated which whirls up activated sludge sedimented on thebottom, homogenizes the content of the SU tank and transports floatingsludge, which may have developed, into the B tank where it canreprocessed into the activated sludge.

A B tank may, e.g., be hydraulically connected with two SU tanks and thecycle times are assumed approx. 140 min: S phase approx. 5 min; R phaseapprox. 5 min; V phase approx. 60 min.; A phase approx. 70 min;A=(S+R+V).

With three SU tanks a cycle of approx. 105 min is obtained: S phaseapprox. 5 min; R phase approx. 5 min; V phase approx. 60 min; A phaseapprox. 35 min; A=(S+R+V):2.

For the outlet of the clear water a firmly mounted air-pressure closurehas proved suitable (FIG. 4). For the surplus and floating sludgeoutlet, also an automatically working air-jet lift may be employed.

Further details of the invention can be taken from the followingdescription, with reference to the drawing. Therein the figures show:

FIG. 1: a schematic representation of the individual phases during acycle,

FIG. 2: a schematic representation of a duplex siphon for the transportof fluids in both directions,

FIG. 3: a schematic representation of the individual phases during acycle using a duplex siphon, and

FIG. 4: a schematic representation of a clear water outlet (air pressureclosure).

FIGS. 1a to 1 d show schematic representations of the phases S, R, V andA. The vertical section along the direction of flow leads through the Btank and one of the at least two SU tanks. The continuous inflow isopposed to an outflow in the A phase only. The S and R phases areoperated with stirring devices in this representation. The near-surfaceopenings are closed in the V and A phases. The activation tank isdenoted with B, and the sedimentation and recirculation tanks with SU.The S phase is illustrated schematically in FIG. 1a. Thickened sludgeQ_(S) is transported from the SU tank into the B tank by means of, inthis case, a stirring apparatus through a permanently open openingsituated near to the bottom, and the same amount Q_(S) flows back viathe opening situated near to the surface from the B tank to the SU tank.In place of a stirring apparatus e.g. a air-jet lift may be employed aswell. FIG. 1b represents the stirring phase. In this case by means of astirring apparatus a strong flow of fluid Q_(R) is generated, whichwhirls up and homogenizes the content of the SU tank. A flow of equalsize comes into the B tank from the SU tank via the surface-nearopenings. In FIG. 1c the V phase can be seen. While in the SU tank thesludge sediments and forms a defined sludge level, the B tank is aeratedin this case with fine-bubble pressured air. Also the surface-nearopenings are closed. Finally, FIG. 1d shows the A phase, in which anoutflow Q_(ab) takes place which corresponds to the inflow Q_(zu). Theopenings near to the surface are closed. An amount of fluid,corresponding to the inflow Q_(zu) and consisting of water and sludge,flows to the SU tank through the permanently open hydraulic connectionat the tank bottom.

In FIG. 2 a duplex siphon is depicted schematically. FIG. 2a shows theoperation in the S phase, and FIG. 2b that in the R phase. In FIG. 2a,by introducing pressured air Q_(L) (the air bubbles are shown) an amountof fluid Q_(S) is transported from the SU tank into the B tank. In FIG.2b an opposite fluid flow from the B tank into the SU tank is produced,wherein Q_(R) is greater than Q_(S). It is also essential that the flowof fluid Q_(R) enters the SU tank with such a high flow velocity (v≈2.0m/s), that sludge sediments on the bottom are whirled up and the contentof the SU tank is homogenized.

FIGS. 3a to 3 d show schematic representations of the phases S, R, V andA with usage of the duplex siphon depicted in FIG. 2. At the side therespective state of the openings positioned near to the surface with theflaps can be seen. For FIGS. 3a to 3 d the discussion of FIGS. 1a to 1 dis substantially applicable as well.

Finally, FIG. 4 shows a possible clear water outlet with air-pressureclosure (air cushion closure). At distances of approximately 1 m,drainage sockets oriented vertically downward are arranged along a tubemounted horizontal. For realizing the closure, pressured air Q_(L) isforced into the horizontal tube. FIG. 4a shows a closed air-pressureclosure, in which a small amount Q_(L) of pressured air, continuouslyinjected, evades through a small pipe in order to maintain a constantwater level difference ΔH. The maximal water level in the SU tank isdenoted with “max.w.l.” and the water level in the outlet channel with“w.l. in outlet channel”. In the air-pressure closure an air pressurecorresponding to the difference ΔH is present. In FIG. 4b an openair-pressure closure is depicted. The amount of outflow is Q. The outletchannel is, in this case, an open drain; it could be realized as apressure pipe as well. Inside and outside of the air-pressure closurethe same air pressure is present.

What is claimed is:
 1. Method for biological purification wastewater bymeans of activated sludge, wherein the wastewater is introduced firstinto an aerated activated sludge tank (B tank) and then, by turns, intoone of several sedimentation and recirculation tanks (SU tanks), whichare permanently hydraulically linked with said B tank and in which,several times a day an operating cycle proceeds comprising a stirringphase (R phase), a pre-settling phase (V phase) and a discharge phase (Aphase), wherein in turns in the R phase the activated sludge is remixedwith the water, in the V phase the activated sludge settles down and inthe A phase clear water is drawn off, and wherein the cycles in the SUtanks are phase displaced to each other, the A phases are adjacent, onlyin the A phases the SU tanks are flown through, an approximatelyconstant water level is present, thus causing an outflow from thepurification plant corresponding to its inflow (throughflow principle),wherein after the V phases and before the R phases settled and thickenedactivated sludge is reintroduced from the SU tank into the B tank (Sphase).
 2. Method of claim 1, wherein the reintroduction of settledactivated sludge occurs during or preferable after the A phase. 3.Method of claim 2, wherein the volume displaced in the B tank during theS phase with less dry substance than the introduced settled activatedsludge is returned via openings near to the surface into the SU tanks,and the surface-near openings allow a throughflow only in the S and Rphases and are closed in the V and A phases.
 4. Method of claim 1,wherein the stirring in the SU tanks (R phase) is done by injecting air,by means of electrically driven stirring apparatuses or by means ofair-jet lifts.
 5. Method of claim 1, wherein for reintroducing settledactivated sludge and for stirring in the SU tanks (S and R phases) acombined air-jet lift (duplex siphon) is used, which allows a transportof fluid in both directions and provides a permanent hydraulicconnection between the B tank and SU tanks in the V and A phases,wherein the stirring effect in the SU tanks is produced by generating astrong jet of water which whirls up activated sludge sedimented on thebottom, generates a water roll with homogenizing effect and transportspossibly developing floating sludge via the surface-near openings intothe B tank.
 6. Method of claim 1, wherein the aeration of the B tank isintermittent in the R phases, in the S phases or in both.
 7. Method ofclaim 1, wherein a B tank is hydraulically connected with two SU tanksand the cycle times are chosen based on approximately 140 min (S phaseapprox. 5 min; R phase approx. 5 min; V phase approx, 60 min; A phaseapprox. 70 min; A=(S+R+V)), or with three SU tanks and the cycle timesare chosen based on approximately 105 min (S phase approx. 5 min; Rphase approx. 5 min, V phase approx. 60 min; A phase approx. 35 min;(S+R+V); 2).
 8. Method of claim 1, wherein an outlet is realized as apneumatic closure having a horizontal tube and at least one drainagesocket oriented downward, wherein into the horizontal tube pressured aircan be injected.
 9. Method of claim 1, wherein at the end of the V phasethe sludge concentration is measured at a predelined depth ofapproximately 1.0 to 1.5 m below the water level, and at the end of theA phase thickened surplus sludge is drawn off for a short time (a fewminutes) if the measurement of the sludge concentration revealed asludge level lying above the measuring position.
 10. Method of claim 1,wherein the B tank is permanently connected hydraulically with the SUtanks via one or more openings in the middle region (at about half thewater depth), in the S phase thickened sludge is led from the bottom ofthe SU tank into the upper region (near to the surface) of the B tankand the content of the B tank thereby displaced is returned via theopenings in the middle region of the tanks. In the R phase the contentof the SU tank is whirled up and homogenized without generation of acirculating current over the B tank and in the A phase the flow from theB tank into the SU tanks occurs via the openings in the middle region aswell.